Thank you for your input! We will consider this functionality in upcoming releases if it will be popular.
Everyone, please continue voting for this feature if you consider it important.
We are planning to make system logs (panel.log, autoinstaller.log, mail server logs, etc) accessible in Plesk UI via Log Browser.
—AKJamin Andrews commented
This would also be useful for wrong user and passwords for end user mail access. This would be our biggest support issue. Many Web Professionals so not know how to SSH and view/tail mail logs. If you could see live log, you could see wrong user and password auth and reseller/web professional could fix issue quickly without our support.